Non-ballistic atmospheric entry is a class ofatmospheric entry trajectories that follow a non-ballistic trajectory by employingaerodynamic lift in the high upper atmosphere. It includes trajectories such as skip and glide.^[1][2]

Skip is a flight trajectory where the spacecraft goes in and out the atmosphere.Glide is a flight trajectory where the spacecraft stays in the atmosphere for a sustained flight period of time.^[1] In most examples, a skip reentry roughly doubles the range ofsuborbital spaceplanes andreentry vehicles over the purely ballistic trajectory. In others, a series ofskips allows the range to be further extended.

Non-ballistic atmospheric entry was first seriously studied as a way to extend the range ofballistic missiles, but was not used operationally in this form as conventional missiles with extended range were introduced. The underlying aerodynamic concepts have been used to producemaneuverable reentry vehicles (MARV), to increase the accuracy of some missiles like thePershing II. More recently, the concepts have been used to producehypersonic glide vehicles (HGV) to avoid interception as in the case of theAvangard. The range-extension is used as a way to allow flights at lower altitudes, helping avoidradar detection for a longer time compared to a higher ballistic path.

The concept has also been used to extend the reentry time for vehicles returning to Earth from the Moon, which would otherwise have to shed a large amount of velocity in a short time and thereby suffer very high heating rates. TheApollo Command Module also used what is essentially a skip re-entry, as did the SovietZond and ChineseChang'e 5-T1.

The conceptual basis was first noticed by German artillery officers, who found that theirPeenemünder Pfeilgeschosse arrow shells traveled much further when fired from higher altitudes. This was not entirely unexpected due to geometry and thinner air, but when these factors were accounted for, they still could not explain the much greater ranges being seen. Investigations atPeenemünde led them to discover that the longer trajectories in the thinner high-altitude air resulted in the shell having anangle of attack that producedaerodynamic lift at supersonic speeds. At the time this was considered highly undesirable because it made the trajectory very difficult to calculate, but its possible application for extending range was not lost on the observers.^[3]

In June 1939, Kurt Patt ofKlaus Riedel's design office at Peenemünde proposed wings for converting rocket speed and altitude into aerodynamic lift and range.^[4] He calculated that this would roughly double range of theA-4 rockets from 275 kilometres (171 mi) to about 550 kilometres (340 mi). Early development was considered under the A-9 name, although little work other thanwind tunnel studies at theZeppelin-Staaken company would be carried out during the next few years. Low-level research continued until 1942 when it was cancelled.^[5]

The earliest known proposal for the boost-glide concept for truly long-range use dates to the 1941Silbervogel, a proposal byEugen Sänger for a rocket poweredbomber able to attackNew York City from bases inGermany then fly on for landing somewhere in thePacific Ocean held by theEmpire of Japan. The idea would be to use the vehicle's wings to generate lift and pull up into a new ballistic trajectory, exiting the atmosphere again and giving the vehicle time to cool off between the skips.^[6] It was later demonstrated that the heating load during the skips was much higher than initially calculated, and would have melted the spacecraft.^[7]

In 1943, the A-9 work was dusted off again, this time under the nameA-4b. It has been suggested this was either because it was now based on an otherwise unmodified A-4,^[5] or because the A-4 program had "national priority" by this time, and placing the development under the A-4 name guaranteed funding.^[8] A-4b usedswept wings in order to extend the range of the V2 enough to allow attacks on UK cities inthe Midlands or to reachLondon from areas deeper within Germany.^[3] The A-9 was originally similar, but later featured longogival delta shaped wings instead of the more conventional swept ones. This design was adapted as a crewed upper stage for the A-9/A-10 intercontinental missile, which would glide from a point over the Atlantic with just enough range to bomb New York before the pilotbailed out.^[8][a]

In the immediate post-war era, Soviet rocket engineerAleksei Isaev found a copy of an updated August 1944 report on theSilbervogel concept. He had the paper translated to Russian, and it eventually came to the attention ofJoseph Stalin who was intensely interested in the concept of anantipodal bomber. In 1946, he sent his sonVasily Stalin and scientistGrigori Tokaty, who had also worked on winged rockets before the war, to visit Sänger andIrene Bredt in Paris and attempt to convince them to join a new effort in theSoviet Union. Sänger and Bredt turned down the invitation.^[10]

In November 1946, the Soviets formed the NII-1design bureau underMstislav Keldysh to develop their own version without Sänger and Bredt.^[11] Their early work convinced them to convert from a rocket powered hypersonic skip-glide concept to aramjet powered supersoniccruise missile, not unlike theNavaho being developed in the United States during the same period. Development continued for a time as theKeldysh bomber, but improvements in conventional ballistic missiles ultimately rendered the project unnecessary.^[10][b]

In the United States, the skip-glide concept was advocated by many of the German scientists who moved there, primarilyWalter Dornberger andKrafft Ehricke atBell Aircraft. In 1952, Bell proposed a bomber concept that was essentially a vertical launch version ofSilbervogel known as Bomi. This led to a number of follow-on concepts during the 1950s, including Robo,Hywards,Brass Bell, and ultimately theBoeing X-20 Dyna-Soar.^[12] Earlier designs were generally bombers, while later models were aimed at reconnaissance or other roles. Dornberger and Ehricke also collaborated on a 1955Popular Science article pitching the idea for airliner use.^[13][14]

The introduction of successfulintercontinental ballistic missiles (ICBMs) in the offensive role ended any interest in the skip-glide bomber concepts, as did thereconnaissance satellite for the spyplane roles. The X-20 space fighter saw continued interest through the 1960s, but was ultimately the victim of budget cuts; after another review in March 1963,Robert McNamara canceled the program in December, noting that after $400 million had been spent they still had no mission for it to fulfill.^[15]

Through the 1960s, the skip-glide concept saw interest not as a way to extend range, which was no longer a concern with modern missiles, but as the basis for maneuverable reentry vehicles for ICBMs. The primary goal was to have the RV change its path during reentry so thatanti-ballistic missiles (ABMs) would not be able to track their movements rapidly enough for a successful interception. The first known example was theAlpha Draco tests of 1959, followed by the Boost Glide Reentry Vehicle (BGRV) test series,ASSET^[16] andPRIME.^[17]

This research was eventually put to use in thePershing II's MARV reentry vehicle. In this case, there is no extended gliding phase; the warhead uses lift only for short periods to adjust its trajectory. This is used late in the reentry process, combining data from aSinger Kearfottinertial navigation system with aGoodyear Aerospace activeradar.^[18] Similar concepts have been developed for most nuclear-armed nations'theatre ballistic missiles.

TheSoviet Union had also invested some effort in the development of MARV to avoid US ABMs, but the closure of the US defenses in the 1970s meant there was no reason to continue this program. Things changed in the 2000s with the introduction of the US'sGround-Based Midcourse Defense, which ledRussia to reanimate this work. The vehicle, referred to asObject 4202 in the Soviet era, was reported in October 2016 to have had a successful test.^[19] The system was revealed publicly on 1 March 2018 as thehypersonic glide vehicle (HGV) Avangard (Russian:Авангард; English:Vanguard), which officially entered active service as an ICBM payload on 27 December 2019.^[20]Vladimir Putin announced that Avangard had entered serial production, claiming that its maneuverability makes it invulnerable to all current missile defences.^[21]

China has also developed a boost-glide warhead, theDF-ZF (known to US intelligence as "WU-14").^[22] In contrast to the US and Russian MARV designs, the DF-ZF's primary goal is to use boost-glide to extend range while flying at lower altitudes than would be used to reach the same target using a purely ballistic path. This is intended to keep it out of the sight of theUS Navy'sAegis Combat System radars as long as possible, and thereby decrease the time that system has to respond to an attack. DF-ZF was officially unveiled on 1 October 2019. Similar efforts by Russia led to theKholod andGLL-8 Igla hypersonic test projects, and more recently the Yu-71 hypersonic glide vehicle which can be carried byRS-28 Sarmat.^[23][24]

Boost-glide became the topic of some interest as a possible solution to the USPrompt Global Strike (PGS) requirement, which seeks a weapon that can hit a target anywhere on the Earth within one hour of launch from theUnited States. PGS does not define the mode of operation, and current studies includeAdvanced Hypersonic Weapon boost-glidewarhead,Falcon HTV-2hypersonic aircraft, and submarine-launched missiles.^[25]Lockheed Martin is developing this concept as the hypersonicAGM-183A ARRW.^[26]

The technique was used by the SovietZond series of circumlunar spacecraft, which used one skip before landing. In this case a true skip was required in order to allow the spacecraft to reach the higher-latitude landing areas.Zond 6,Zond 7 andZond 8 made successful skip entries, althoughZond 5 did not.^[27][28] TheChang'e 5-T1, which flew mission profiles similar to Zond, also used this technique.

TheApollo Command Module used a skip-like concept to lower the heating loads on the vehicle by extending the re-entry time, but the spacecraft did not leave the atmosphere again and there has been considerable debate whether this makes it a true skip profile.NASA referred to it simply as "lifting entry". A true multi-skip profile was considered as part of the Apollo Skip Guidance concept, but this was not used on any crewed flights.^[29] The concept continues to appear on more modern vehicles like theOrion spacecraft, which made the first American skip entry in theArtemis 1 mission, using onboard computers.^[30][31][32]

Using simplified equations of motion and assuming that during the atmospheric flight both drag and lift forces will be much larger than the gravity force acting on the vehicle, the following analytical relations for a skip reentry flight can be derived:^[33]

$${\displaystyle {\gamma }_{\mathrm{F}}=-{\gamma }_{\mathrm{E}},}$$

where${\displaystyle \gamma }$ is the flightpath angle relative to the local horizontal, the subscript E indicates the conditions at the start of the entry and the subscript F indicates the conditions at the end of the entry flight.

The velocity${\displaystyle V}$ before and after the entry can be derived to relate as follows:

$${\displaystyle \frac{V_{\mathrm{F}}}{V_{\mathrm{E}}}=\exp \frac{2{\gamma }_{\mathrm{E}}}{L/D},}$$

where${\displaystyle L/D}$ is thelift-to-drag ratio of the vehicle.

• High Speed Strike Weapon (HSSW) (USA)
• Prompt Global Strike (PGS) (USA)
• Alpha Draco (USA)
• ArcLight (missile) (USA)
• DARPA Falcon Project (USA)
• Boeing X-51 Waverider (USA)
• North American X-15 (USA)
• Tupolev Tu-130 (Russia)
• BrahMos-II (India / Russia)
• Hypersonic Technology Demonstrator Vehicle (India)
• Multiple independently targetable reentry vehicle
• Planar reentry equations

• Neufeld, Michael (1995).The Rocket and the Reich: Peenemünde and the Coming of the Ballistic Missile Era. Simon and Schuster.ISBN 9780029228951.
• Yengst, William (April 2010).Lightning Bolts: First Maneuvering Reentry Vehicles. Tate Publishing.ISBN 9781615665471.

• Cancelled military aircraft projects of the United States
• Atmospheric entry
• Apollo program
• Spaceplanes
• Space weapons
• Hypersonic aircraft

• CS1 maint: bot: original URL status unknown
• CS1 Russian-language sources (ru)
• Articles with short description
• Short description with empty Wikidata description
• Articles containing Russian-language text
• Articles containing explicitly cited English-language text